Such transducers are typically piezoelectric in operation, with one or more piezoelectric elements and associated contact electrodes clamped between loading blocks to provide a relatively high-Q assembly that will oscillate at a predetermined frequency when excited by an alternating electric potential at that frequency so as to transmit acoustic energy. The same transducer is commonly used to receive reflected acoustic energy at that frequency and convert it back to electrical energy.
Since the potentials required to excite such transducers are typically quite high, and they are high impedance devices, and may need to be located some distance from a transceiver which generates the excitation signal and processes received signals, they are usually associated with an impedance matching transformer and possibly also with temperature sensing components and preamplifying or preprocessing circuits for received signals. Furthermore, it is usually necessary to provide acoustic matching between the transducer assembly and a surrounding medium, usually gaseous, to provide the transducer assembly with suitable directional properties, to protect the transducer assembly from the surrounding medium, and to isolate the transducer assembly as far as possible from the structure on which it is mounted.
The above requirements must be accommodated by the enclosures applied to house such transducers. Typically, the transducer assembly is wrapped on its non-radiating surfaces with a material such as cork, and placed in a moulded or fabricated metallic or moulded plastic shell which may be selectively lined with material such as cork and is then filled with a potting compound so as to embed the transducer assembly. The shell may be open on a side corresponding to the radiating surface of the transducer, in which case a layer or layers of acoustic matching material may be cast into the shell so as to cover the radiating surface of the assembly, or it may be closed by a thin diaphragm at that surface, in which case matching material must be installed within the diaphragm prior to inserting and potting the transducer. In either case, the assembly process is slow, laborious, labour intensive, and must be carefully controlled so that components are properly located within the shell. The installation of transformers and other electronic components further complicates the process.
Typical transducers are described in U.S. Pat. Nos. 3,674,945 (Hands) and 5,339,292 (Brown et al).